As is known, for performing complex processing functions, analog memories with cells capable of memorizing analog or digital multibit values are currently being studied, for which use is advantageously made of nonvolatile floating-gate cells, particularly flash-EEPROMs, in which the value to be memorized is related to the threshold voltage of the cell. To read the cells and/or perform calculating operations, a number of voltage references are required; the references must be programmable to achieve a high degree of flexibility as regards use of the memories, to enable uniform design of memories to be used for various applications and in particular for various analog values to be stored, and to enable the reference voltages to be modified even during operation of the memory, according to the functions to be performed.
According to one known solution described in U.S. Pat. No. 5,126,967, the reference (and data) cells are read via a sample-and-hold circuit. This solution, however, fails to provide for fast read times when memorizing a large number of voltage levels.